Air flow rate measuring devices that measure a flow rate of an intake air for an internal combustion engine have been known. Such devices may include an attachment portion attached to a duct and a casing that extends from the attachment portion in a vertical direction into an inside of the duct. The casing houses a sensor to detect a flow rate of the intake air.
To improve accuracy of detecting a flow rate, a sensor chip including a plurality of film resistors arranged on a surface of a semiconductor substrate has been used in recent years.
In the duct, foreign substances such as dusts, which were not removed by an air cleaner, may be mixed in the intake air and be taken in the casing. If such foreign substances collide with the sensor chip, the resistors of the sensor chip may be damaged.
In view of this, some air flow rate measuring devices include a main-bypass passage and a sub-bypass passage.
The main-bypass passage takes in an intake air from the duct, and then discharges the intake air into the duct again. The sub-bypass passage branches off form the main-bypass passage and takes in a portion of the intake air from the main-bypass passage, and then returns the portion of the intake air into the main-bypass passage. The sensor chip is housed in the sub-bypass passage (see Patent Literature JP 2004-012274 A).
Hence, a major portion of foreign substances taken in through the inlet together with the intake air flows through only the main-bypass passage without flowing through the sub-bypass passage 12 and is discharged through the outlet 14 into the duct 2 by inertial force.
However, if an air cleaner having a low capturing power were used, or if a vehicle were traveling in an environment containing many dusts, it would be necessary to reduce an amount of foreign substances flowing into the sub-bypass passage from the main-bypass passage.
In view of the above, it is an objective of the present disclosure to provide an air flow rate measuring device that is capable of reducing an amount of foreign substances flowing into the sub-bypass passage from the main-bypass passage.